The recent trend toward higher integration in the integrated circuit technology poses a demand for finer feature size patterns. In processing of patterns with a size of less than 0.2 μm, acid-catalyzed chemically amplified resist compositions are used in most cases. The light source for exposure in the processing is high-energy radiation including UV, deep UV, electron beam (EB), X-ray, excimer laser, γ-ray, and synchrotron radiation. Among others, the EB lithography is utilized as the ultrafine processing technology and in particular, indispensable as the process for forming a pattern on a photomask which becomes an original image for pattern exposure.
The photomask resulting from EB writing offers an original image for the fabrication of semiconductor devices. For mask pattern formation, the accuracy of EB writing position and the accurate control of pattern line width are critical.
One of problems inherent to the EB lithography is the charge-up phenomenon that electric charges accumulate on or in the resist film during exposure. The charge causes the path of incident EB to be deflected, substantially detracting from the accuracy of mask pattern writing. The phenomenon may be avoided by coating an antistatic film on the resist film so that the charge may be released. The antistatic means is indispensable for writing fine size patterns.
However, another problem arises when an antistatic film is coated on a chemically amplified resist film. The acid in the antistatic film diffuses into the resist film, whereby noticeable changes of line width, shape and sensitivity occur after exposure. Also, the acid generated in the resist film by exposure is neutralized with a certain component in the antistatic film, whereby changes of line width and sensitivity similarly occur, failing in accurate writing.
Since the resist film is hydrophobic on its surface, it has no affinity to aqueous antistatic agents. It is thus difficult to coat the antistatic agent onto the resist film. As a solution to this problem, JP-A 2002-226721 proposes to add a surfactant to facilitate coating operation. This is still unsatisfactory in that some surfactants have detrimental impacts such as intermixing with the resist film surface.
On the other hand, JP-A 2006-048029, for example, discloses that a fluorinated polymer, when used in a resist composition subject to the immersion lithography, is effective for preventing any components in the resist film from being leached out from its surface. The immersion lithography intends to acquire high resolution performance by increasing the incident angle of light, and is a technique of repeatedly transferring the pattern of the photomask as the original to a resist film on a recipient such as wafer.
The immersion lithography is not applicable when a photomask is prepared from a photomask blank by scanning the resist film on the blank directly with a beam of high-energy radiation. Therefore, the fluorinated polymers are not used in the resist material for photomask blanks. JP-A 2008-304590 discloses that a polymer comprising recurring units having fluorine atoms is added to a resist material for photomask blanks to improve resist performance. Even when such a fluorinated polymer is used, the coating of an antistatic film is still inefficient. It is then difficult to meet all factors including resolution and age stability of a resist film and effective coating of an antistatic film.